Add test-tube-only vector file

This PR should remove the need for the changes @penibelst suggested in #4. It adds a separate vector file for just the test-tube part of the logo. The downside is that this means that we’d need to maintain changes to it in two seperate source files, however it saves having to worry about technical workarounds to extract files—it’ll be much easier to just open up the dedicated vector in Illustrator and export the assets you need at whatever sizes are required. The vector here is set up with a 512px square canvas, with the test tube directly centered vertically and horizontally, so should be pretty flexible.

@parkr Your call if you want to proceed this way or the manual extraction stuff suggested in #4.